1. Field of the Invention
The present invention relates to variable height columns, and more specifically, to fluid actuated columns.
2. Description of the Prior Art
Commercially available load support surfaces, such as surgical tables, may include variable height columns which can be raised and lowered by fluid actuated cylinders. One type of cylinder includes a sliding way system with a single acting cylinder separate from the column. The separate external cylinder provides a bulky arrangement. Another commercially available column includes a concentric sliding way system with an integral single acting hydraulic cylinder as the motive force. The internal cylinder, with its piston and rod, requires finely finished internal surfaces to prevent excessive friction when raising or lowering the column. The manufacturing costs for such finely finished materials are high.
The conventional columns must be positioned near the center of gravity of the load support surface and its load to limit the bending moment on the column. Excessive bending moments on the columns lead to early column failure. The friction force of the way system is proportional to the bending moment and the coefficient of friction of the materials. A low coefficient of friction will permit a much larger bending moment before the column fails. Conventional sliding way system columns have high coefficients of friction.
Columns having rolling element way systems have relatively low coefficients of friction and thus, will tolerate greater bending moments, but are limited by contact stress between the rollers and the track of the way systems. Furthermore, rolling element systems are bulky.
In order to increase the extension of columns, two stage motive forces have been designed. The rate of motion, however, from one stage to the next is not uniform. Greater weight must be lifted in the first stage, thus, the rate of motion is relatively slow. The second stage is more rapid. When raising a patient support surface, a uniform rate of motion is preferred.
It is an object of the present invention to provide a variable height column which permits a streamlined, easily manufactured structure by providing an internal motive force which does not require finely finished internal surfaces. It is a further object of the present invention to provide a variable height column which has a uniform rate of motion and a large extended to collapsed ratio.
Finally, it is an object of the present invention to provide a column having a low coefficient of friction with a high load bearing capacity to permit greater tolerance to bending moments so that the column can be positioned away from the center of gravity of the load support surface and its load.